1. Technical Field
The present invention relates to an apparatus and a method for evaluating distortion based on three-dimensional measurement data obtained from a measurement object surface.
2. Background Art
In a body surface such as a surface of a door panel of a motorcar manufactured with using a steel plate, there sometimes can develop a shape different from an originally designed shape (i.e. distortion), due to the thickness and/or composition of the steel plate used. And, determination of whether the distortion is within an acceptable range or not is effected through sensory evaluation by a skilled artisan. However, this determination cannot be made appropriately under a predetermined standard, unless the artisan is an experienced one who has actually observed various distortions. For this reason, there has been proposed a distortion evaluating apparatus designed to extract mechanically a certain characteristics from the distortion in the measurement object surface such as a body surface, thereby effecting the sensory evaluation of distortion degree in a quantitative manner.
According to a technique employed by a distortion evaluating apparatus described in Patent Document 1, a secondary differential operation is carried out on two-dimensional measurement data of a cross section of the measurement object surface, indicative of unevenness of the surface and then e.g. a difference value between the maximum value and the minimum value of the secondary differential values, as the characteristics indicative of the distortion. Thereafter, this difference value is assigned into a prediction formula, by which a distortion evaluation value is predicted. More particularly, a difference between a cross section shape line of the measurement object surface actually determined and an ideal curve of values which comprise e.g. design data per se is calculated, thus obtaining a surface distortion line including all large and small distortions which have developed in the measurement object surface. Then, a secondary differential operation is effected on this surface distortion line obtained with including all large and small distortions, and a difference value between the maximum value and the minimum value of the resultant secondary differential values is utilized for the distortion evaluation. The secondary differential values of this two-dimensional measurement data (surface distortion line) correspond to curvature data of the cross section of the measurement object surface. And, it may be determined that the greater the absolute value of the secondary differential value, the greater the distortion.
Patent Document 1: Japanese Patent No. 3015615
According to the distortion evaluating apparatus described in Patent Document 1, the apparatus employs a difference value between the maximum value and the minimum value of two-dimensional measurement data, as a characteristics used for distortion degree evaluation. This means that the apparatus employs data including all, i.e. large and small distortions present in the measurement object surface, for the purpose of distortion evaluation of the measurement object surface. In the case of a sensory evaluation by a human, on the other hand, even when a distortion exists, this may sometimes be determined as permissible if it is not conspicuous. On the other hand, in the case of the conventional method using the secondary differential values of two-dimensional measurement data as they are, without effecting any data processing thereon, the method is configured to take note of and find problematic even such small distortion also which would be found permissible by a sensory evaluation by a human.
Further, if the absolute value of a secondary differential value is large, this should be determined appropriately as being indicative of a large distortion. However, such appropriate evaluation may sometimes be not done by the distortion evaluating apparatus disclosed in Patent Document 1. For instance, a certain waveform can have a large peak (maximum value) of absolute value in the positive direction and has a small peak (minimum value) of absolute value in the negative direction. Another waveform can have equal peaks (maximum value and minimum value) of absolute value in the positive direction and negative direction. Still another waveform can have a small peak (minimum value) of absolute value in the positive direction and has a large peak (maximum value) of absolute value in the negative direction. In such case, the distortion evaluating apparatus described in Patent Document 1 would provide a same distortion evaluation result for all of these three kinds of waveforms as long as the difference value between the maximum value and the minimum value of the secondary differential values is the same.
Therefore, the distortion evaluation result obtained by the conventional distortion evaluating apparatus would be different from the desirable result which could be obtained by the human sensory evaluation. Hence, it cannot be said that this conventional apparatus really effects quantitative evaluation of distortion in a measurement object surface.
Moreover, the original shape of the measurement object surface to be used as the baseline can sometimes be deformed by the spring-back phenomenon of the steel plate, so that the cross section line obtained therefrom may deviate from the design data. In this case, the conventional method would determine such original shape of the measurement object surface too as “distortion”, which actually is not. More particularly, the distortion evaluating apparatus disclosed in Patent Document 1 is configured to attempt to calculate difference values between a cross section shape line and an ideal curve so as to obtain a surface distortion line which includes only the distortions which have developed in the measurement object surface. As a matter of fact, the above difference values include not only the distortions, but also the original shape of the measurement object surface. Moreover, even if no distortion has actually developed, there is the possibility of the original shape of the measurement object surface formed by the spring back being determined as a distortion erroneously.
In addition, in the case of the distortion evaluating apparatus described in Patent Document 1, the apparatus monitors the secondary differential values and calculates a difference value between the maximum and minimum values thereof. Hence, there is another problem of the apparatus being constantly under a computational load over a predetermined level.
The present invention has been made in view of the above-described problem. And, its object is to provide an apparatus and a method for evaluating distortion which can quantitatively evaluate distortion in a measurement object surface.